Jupiter's second largest moon, Callisto, may have a liquid
ocean tucked under its icy, cratered crust, according to
scientists studying data gathered by NASA's Galileo spacecraft.

The Galileo findings, to be published in the Oct. 22 issue
of the journal Nature, reveal similarities between Callisto and
another of Jupiter's moons, Europa, which has already displayed
strong evidence of a subsurface ocean.

"Until now, we thought Callisto was a dead and boring moon,
just a hunk of rock and ice," said Dr. Margaret Kivelson, space
physics professor at UCLA and principal investigator for
Galileo's magnetometer instrument, which measures magnetic fields
around Jupiter and its moons. "The new data certainly suggest
that something is hidden below Callisto's surface, and that
something may very well be a salty ocean."

This premise was inspired by Galileo data indicating
electrical currents flowing near Europa's surface cause changes
in Europa's magnetic field. "This seemed to fit nicely with
other data supporting the idea that beneath Europa's icy crust, a
liquid ocean might be serving as a conductor of electricity,"
said Kivelson.

Armed with that information, Kivelson and UCLA colleagues
Drs. Krishan K. Khurana, Raymond J. Walker, and Christopher T.
Russell set out to test a similar theory about Callisto,
"although it seemed far-fetched at the time," Kivelson said. The
team went back and studied data obtained during Galileo's flybys
of Callisto in November 1996, and June and September of 1997.

Kivelson and her colleagues found signs that Callisto's
magnetic field, like Europa's, is variable, which can be
explained by the presence of varying electrical currents
associated with Jupiter that flow near Callisto's surface. Their
next challenge was to discover the source of the currents.

"Because Callisto's atmosphere is extremely tenuous and
lacking in charged particles, it would not be sufficient to
generate Callisto's magnetic field; nor would Callisto's icy
crust be a good conductor, but there very well could be a layer
of melted ice underneath," Kivelson said. "If this liquid were
salty like Earth's oceans, it could carry sufficient electrical
currents to produce the magnetic field."

Lending further credence to the premise of a subsurface
ocean on Callisto, Galileo data showed that electrical currents
were flowing in opposite directions at different times. "This is
a key signature consistent with the idea of a salty ocean,"
Khurana added, "because it shows that Callisto's response, like
Europa's, is synchronized with the effects of Jupiter's
rotation."

Although scientists consider the possible presence of an
ocean on Europa as one factor hinting that life could have
developed there at some point, it is doubtful that Callisto could
harbor life, according to Galileo Project Scientist Dr. Torrence
Johnson of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.

"The basic ingredients for life -- what we call 'pre-biotic
chemistry' -- are abundant in many solar system objects, such as
comets, asteroids and icy moons," Johnson explained. "Biologists
believe liquid water and energy are then needed to actually
support life, so it's exciting to find another place where we
might have liquid water. But, energy is another matter, and
currently, Callisto's ocean is only being heated by radioactive
elements, whereas Europa has tidal energy as well," from its
greater proximity to Jupiter.

Galileo flies by Callisto four more times between May and
September of 1999, which may yield more clues about the
possibility of a Callisto ocean. However, Kivelson said that
scientists will rely heavily on theoretical models to test their
interpretations about Callisto.

Kivelson and her team also are reexamining magnetometer data
from Jupiter's largest moon, Ganymede, to address the tantalizing
concept that Callisto and Europa may not be the only moons of
Jupiter with subsurface oceans.

The latest Galileo exterior images of Callisto, released on
Oct. 13, and a new artist's concept of a cutaway view of the
moon's interior are available on the Internet at the following
websites:

Galileo has been in orbit around Jupiter, studying the huge
planet, its moons and its magnetic environment, for more than 2 ½
years. It is currently in the midst of a two-year extension
known as the Galileo Europa Mission. Galileo is managed by JPL
for NASA's Office of Space Science, Washington, DC. JPL is a
division of Caltech, Pasadena, CA.

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NOTE TO EDITORS: Interview clips with Dr. Margaret Kivelson, as
well as images of Callisto, will be transmitted on NASA
Television on Wed., Oct. 21, 1998 at 9 a.m., 12 p.m., 3 p.m., 6
p.m. and 9 p.m. Pacific Time. NASA Television is available on
the satellite GE-2, transponder 9C, at 85 degrees west longitude,
vertical polarization, frequency 3880 Mhz, audio 6.8 MHz.